A gas turbine engine is typically mounted below an aircraft wing or within an aircraft tail section to a pylon. The engine is typically mounted at both its forward end and at its aft end for transmitting loads to the pylon. The loads typically include vertical loads such as the weight of the engine itself, axial loads due to the thrust generated by the engine, side loads such as wind buffeting, and roll loads or moments due to rotary operation of the engine. The mounts must also accommodate both axial and radial thermal expansion and contraction of the engine relative to the supporting pylon.
In one type of aft mount, a support bracket is fixedly joined to the pylon by two spaced pins in a plane extending perpendicularly through the centerline of the engine, and to a turbine frame in the engine by a third pin in an L-shaped configuration. Spaced circumferentially from the bracket is a simple link which is pivotally joined to both the pylon and the frame. The bracket is provided so that the aft mount assembly can accommodate in-plane loads, i.e. those in a single vertical axial plane extending perpendicularly to the engine longitudinal centerline axis, including the vertical loads, side or horizontal loads, and roll loads or moments, and, therefore, the bracket does not rotate. By rigidly joining the bracket to the pylon at two points and to the frame at a single point, the bracket can transfer in-plane loads from the frame to the pylon through tension, compression, and bending of the bracket.
The link, however, by being pivotally joined between the pylon and frame can only transfer tensile and compressive loads along its longitudinal axis between its mounting pins. The link is otherwise free to rotate in-plane relative to the pylon and frame. Allowing the link to rotate is required for accommodating radial expansion and contraction of the frame without introducing additional reaction stresses which would otherwise occur if the link connection to the frame were prevented from moving relative to the bracket connection to the frame during thermal expansion and contraction.
This exemplary aft mount may further include a failsafe bracket disposed between the support bracket and link which is normally not a load bearing member, but is provided solely for carrying loads upon failure of either the link or support bracket. The failsafe bracket is fixedly joined at its proximal end to the pylon and includes an aperture at its distal end through which is positioned a pin fixedly joined to the frame. A predetermined clearance is provided between the pin and the aperture so that during normal operation of the mount, no loads are transferred from the frame to the pylon through the failsafe bracket. However, upon failure of either the link or the support bracket, the pin will contact the failsafe bracket at its aperture for transferring loads through the failsafe bracket from the frame to the pylon which would otherwise be transmitted through the failed member. The failsafe bracket is fixedly joined to the pylon and does not rotate to ensure that it also can transfer all in-plane loads including vertical and horizontal loads. This is required because the link is allowed to pivot and is, therefore, unable to transfer all in-plane loads if the support bracket were to fail.
This exemplary failsafe bracket is relatively large and heavy for accommodating the required in-plane loads therethrough, and also affects the ability to obtain a compact aft mount which must typically fit within a limited envelope between the engine and the pylon without adversely affecting the airflow over the outer surfaces of the engine nacelle and the pylon fairing.
Furthermore, in order to fit the aft mount in this limited envelope, the distance between the two pins of the support bracket joined to the pylon is typically shorter than the distance between the pin of the support bracket joined to the frame and the middle pin of the support bracket on the pylon. Accordingly, a transverse load acting on the pin at the frame bends the support bracket about the middle pin and results in an amplified reaction couple shear load acting on the middle pin which must be suitably accommodated, by providing a larger diameter pin, for example, to ensure acceptable life.